The invention relates to stretching magnetic materials for use as sputtering targets to increase the pass through flux (PTF) of the magnetic material being sputtered and to decrease the permeability of the magnetic material.
The PTF of a magnetic sputtering target is defined as the ratio of transmitted magnetic field to the applied magnetic field. A PTF value of 100% is indicative of a non-magnetic material where none of the applied field is shunted through the bulk of the target. The PTF of magnetic target materials is typically specified in the range of 0 to 100%, with the majority of commercially produced materials exhibiting values between 10 to 95%. However, the PTF can be also expressed as an absolute value of the transmitted field in units of gauss instead of a percent.
There are several different techniques for measuring product PTF. One technique involves placing a 4.4xc2x10.4 kilogauss bar magnet in contact on one side of the target material and monitoring the transmitted field using an axial Hall probe in contact with the other side of the target material. The maximum value of the magnetic field transmitted through the bulk of the target, or the maximum value transmitted, divided by the applied field strength in the absence of the target between the magnet and probe, which is maintained at the same distance apart as when the target was between them, is defined as the PTF. Another technique for measuring PTF involves using a horseshoe magnet and a transverse Hall probe.
The PTF values measured using different magnet and probe arrangements are found to exhibit good linear correlation for the values of magnet field strength typically utilized in the industry. The PTF measurement techniques are constructed to realistically approximate the applied magnetic flux occurring in an actual magnetron sputtering machine. Therefore, PTF measurements have direct applicability to a target material""s performance during magnetron sputtering.
Magnetron cathode sputtering involves the arrangement where permanent magnets or electromagnets are positioned behind a target material (cathode) and applying a magnetic field to the target. The applied magnetic field transmits through the target and focuses a discharge plasma onto the front of the target. The target front surface is atomized by an ion beam with subsequent deposition of atoms from the target material onto the surface of a substrate positioned adjacent to the target to form a thin film on the substrate.
The use of magnetron sputtering to deposit thin films of magnetic target materials is wide spread in the electronics industry, particularly in the fabrication of semiconductor and data storage devices. Due to the magnetic nature of the target materials, there is considerable shunting of the applied magnetic field in the bulk of the target. This in turn results in reduced target utilization due to focussing of the transmitted magnetic field in the erosion groove formed as a result of the shunting. This focussing effect is exacerbated with increasing material permeability which corresponds to decreasing material PTF.
It is well known that reducing target material permeability or increasing the target material PTF promotes less severe erosion profile, thus enhancing target material utilization during the sputtering process. This leads to a net reduction in target material cost per unit sputter fabricated product. Furthermore, the presence of severe target erosion profiles can also lead to a point source sputtering phenomena which can result in a deposited thin film that lacks thickness uniformity. Therefore, in addition to less severe erosion profile, increasing the PTF of the target material has the added benefit of increasing the uniformity of the thickness of the deposited thin film.
Magnetic material PTF and permeability (i.e., the ratio of magnetic flux density produced in a medium to the magnetizing force producing it) are not mutually exclusive. Rather, there is a very strong inverse correlation between PTF and maximum permeability of magnetic material. Values of material magnetic permeability can be very precisely determined using a vibrating-sample-magnetometer (VSM) technique in accordance with ASTM Standard A 894-89. Descriptions of sample geometry and calculation of the appropriate demagnetization factors for permeability determination are well known in the art. See, for example, Bozarth, Ferromagnetism, p. 846.
Magnetic target PTF is a strong function of both target chemistry and the thermomechanical techniques utilized during target fabrication. For alloys that do not possess inherently high PTF as a result of their stoichiometry, i.e., PTF less than 85%, it is possible to increase product PTF by various thermomechanical manipulations during product fabrication. For example, the typical fabrication of Ni, Co and Co-alloy targets involves casting, hot-rolling and either heat treatment or cold-rolling or a combination of heat treatment followed by cold-rolling. It is known that heat treating and cold-rolling of magnetic target materials can increase product PTF. Heat treatment of Co-Cr-Ta-(Pt) alloys below 2200xc2x0 F. has been shown to increase the PTF by promoting matrix crystallographic phase transformation from face centered cubic to hexagonal closed packed. Chan et al., Magnetism and Magnetic Materials, Vol. 79, pp. 95-107 (1989). It is suggested in Weigert et al., Mat. Sci. and Eng., A 139, pp 359-363 (1991), that cold-rolling of an alloy comprising 62-80 atomic % Co, 18-30 atomic % Ni and 0-8 atomic % Cr immediately after the hot-rolling step results in an increase in product PTF. A similar result is disclosed in Uchida et al., U.S. Pat. No. 5,468,305 for an alloy containing 0.1-40 atomic % Ni, 0.1-40 atomic % Pt, 4-25 atomic % Cr and the remainder Co which is cold-rolled by not more than a 10% reduction after the hot-rolling process. Uchida et al. claim that the cold-deformation induced internal strain in the alloy reduces magnetic permeability.
U.S. Pat. No. 1,586,877 discloses heating treating a nickel-iron alloy and then placing it under tension to stabilize permeability. The patentee discloses that the relationship between permeability and tension for any particular nickel-iron alloy is dependent on the heat treatment. If a wire of about 78% nickel and about 22% iron is mechanically worked and then annealed at a temperature of 800xc2x0 C. for a few minutes and allowed to cool in air, the alloy shows a rapid decrease of permeability with tension when measured with a field strength of 0.01 gauss whereas a similar wire heat treated at 1100xc2x0 C. and measured at the same field strength shows a tremendous increase of permeability with tension.
U.S. Pat. No. 1,801,150 discloses a process of treating magnetic materials such as a nickel-iron alloy to increase the consistency of permeability of the magnetic material by a process wherein the material is successively annealed and elongated. The example in the patent discloses annealing an iron-nickel alloy in bar form at 800xc2x0 to 900xc2x0 C. for about an hour, then elongating the bar to such an extent that a cross-section of the bar is reduced by 10%; annealing the bar for an hour at 900xc2x0 C., again elongating the bar while cold; and so on. The annealing and elongation steps are repeated three times. After the third annealing, the alloy is elongated in a cold state until the cross-sectional reduction amounts to about 60-79%. The resulting product has magnetic stability.
U.S. Pat. No. 4,053,331 discloses that the magnetic permeability of amorphous metallic alloys are improved by application of stress. In this patent, alloys in ribbon form are subjected to controlled tensile stress. The advantages of this process include low field properties and permeability which exceed those of permalloys.
The goal in each of these U.S. patents is to use tension to increase permeability, i.e., decrease PTF, of soft Ni-Fe alloys. This goal is opposite to that desired with respect to magnetic target alloys used in magnetron sputtering. The inventors have discovered that the PTF of such magnetic target alloys can be increased and the permeability decreased by using the combined steps of hot rolling and stretching the alloy material. These combined steps are not disclosed, taught or suggested by the prior art.
The object of this invention is to provide a process for preparing a magnetic material for use as a target for magnetron cathode sputtering and depositing thin films of magnetic material having increased PTF and decreased permeability. It is a further object of the present invention to provide a sputtering target for use in magnetron cathode sputtering which is prepared by stretching. It is a further object of the invention to provide sputtered thin magnetic films having uniform thickness. To accomplish the object described above according to the present invention, there is provided a process of hot rolling the magnetic material and then stretching it at least 3% and up to 16%. Other objects and characteristics of the present invention will become apparent from the further disclosure of the invention which is given hereinafter with reference to the accompanying drawing.